I've looked in Longstreet's papers, the Official Records, EP Alexander's autobiography, a couple of private soldiers' diaries and Peter Cozzen's "This Terrible Sound," and cannot find any mention of a train wreck. I would think that since there was only one open railroad they could have taken any wreck would have been a significant emotional event that would have been noted somewhere. Are you thinking of a different campaign maybe? Sorry I couldn't help more.

I don't know of any train wreck but the line Longstreet wanted too use was torn up
and could not be used. This sent him on a long detour making him late getting to the battle
although his men were already there.

The last minute change of rail lines caused General Longstreet's corps to have to
travel over sixteen different railroads. The railroads that General Longstreet used had two
different gauges and one bottleneck point for the switch over. More delays stemmed from
Southern cities not allowing railroads to enter the city limits. Some city officials believed
steam engines could spark causing unnecessary fires as they passed through towns. An
example of this gap was the one at Augusta, Georgia, where there was a gap of less than a
half mile. The men would disembark, march through town, and then reload or wait until the
trains returned from a previous run. At other points, the men would have to ferry across
rivers to reach the other line. These issues posed problems, and caused serious delays and
bottlenecks along the way.

To move his troops, General Longstreet had to rely on the Richmond rail organizations to
organize the different rail systems. The Confederate railroads were run by an excellent
administrator, Major Frederick W. Sim's. He had served in an infantry regiment that
surrendered at Ft. Pulaski and spent time in a prisoner of war camp before he was released.
Quartermaster General Lawton arranged the movement, while Major Sims, chief of the
Railroad Bureau, planned the route. Lawton described the operation as, "everything turned
on the question of transportation and supply, and it all had to be decided and performed
with telegraphic haste."[13] To minimize bottlenecks, Major Sim's quickly patched together a
circuitous 950-mile route to Chattanooga through Atlanta. Sim's divided the rail traffic
between the Raleigh-Charlotte-Columbia line and the Wilmington-Florence line.[14] This
would put the troops on parallel tracks until they reached Atlanta. From Atlanta, the
Western and Atlantic railroad was a single line to Catoosa Station. A benefit of
Chattanooga's fall was there was no civilian traffic on the Western and Atlantic to slow the
movement even more.

Railroads across the Confederacy were experiencing shortages in rolling stock. The railroads
where operating under these shortages due to manpower issues. Rolling stock could not be
repaired, as the tracks and locomotives where given a higher priority on the repair list.
Rolling stock was loaned to other railroads to help out and never returned to original
owners. The troops traveled in all types of modified vehicles; they rode on flat cars, on top
of and in boxcars, and few rode inside coaches. When soldiers boarded the rolling stock,
they immediately modified them. One soldier, Augustus Dickert, a historian in Kershaw's
brigade, described the rail car modifications, "they were little more than skeleton cars; the
weather being warm the troops cut all but the frame work loose with knives and axes, to
view the fine country and delightful scenery."[15] Moxley Sorrell described the rail cars as
"crazy cars- passenger, baggage, mail, coal, box, platform, all and every sort wabbling on
the jumping strap-iron….[16]